Low blood pressure (hypotension)

Low blood pressure - sometimes referred to as hypotension - is a condition where the arterial blood pressure is abnormally low. Blood pressure is a measure of the force that your heart uses to pump blood around your body. 

The heart

The heart is a muscle that is designed to pump a constant supply of blood around the body. When your heart beats, it pushes the blood around your body through blood vessels called arteries and capillaries. When your heart rests in between beats, the blood flows back to your heart through a network of veins and capillaries. 

Blood pressure 

Blood pressure is a measure of the force of the blood on the walls of the arteries as the blood flows through them. It is measured in millimetres of mercury (mmHg).

When your blood pressure is measured, two measurements are recorded during a single heartbeat. The two measurements are known as the systolic pressure and the diastolic pressure.

• systolic pressure - is the pressure when your heart beats and squeezes blood into your arteries. At this stage, the pressure in your arteries is at its highest. 
• diastolic pressure - is the pressure when your heart rests in between beats and the blood flows back to your heart through your veins. At this stage, the pressure in your arteries is at its lowest

Your blood pressure reading will be given as two numbers, with your systolic reading first, followed by your diastolic reading. If your systolic blood pressure is 120 mmHg, and your diastolic blood pressure is 80 mmHg, your blood pressure is 120 over 80, which is commonly written as 120/80.

The highs and lows

As a general guide, the ideal blood pressure for a young, healthy adult is between 90/60 and 120/80. If you have a reading of 140/90, or more, you have high blood pressure (hypertension). This puts you at greater risk of serious health conditions, such as strokes or heart attacks.

Low blood pressure is also known as hypotension. People with a reading of around 90/60, or less, are commonly regarded as having low blood pressure. Some people who have low blood pressure experience symptoms as a result of it. There may be an underlying cause that could need treatment.

On its own, low blood pressure (hypotension) does not always cause symptoms. If you have low blood pressure and you do not have any symptoms, you do not require treatment.

However, low blood pressure can sometimes mean that there is not enough blood flowing to your brain and other vital organs. As a result, you may experience some of the following symptoms:

• dizziness 
• fainting (a sudden, temporary loss of consciousness)  
• light-headedness 
• blurred vision 
• palpitations (a rapid or irregular heartbeat)
• confusion 
• nausea (feeling like you are going to be sick)
• general weakness

If you experience the symptoms of hypotension after changing positions (for example, standing up), it is known as postural - or orthostatic - hypotension. If you experience these symptoms after eating, it is known as postprandial hypotension.

Postural or orthostatic hypotension

Postural or orthostatic hypotension occurs when your blood pressure falls after a sudden movement. For example, you may feel dizzy or faint after changing posture, such as sitting up from a lying position, or standing up from a sitting position. This may cause you to lose your balance and fall over. You may also feel light-headed, have blurred vision, or lose consciousness.

The symptoms of postural or orthostatic hypotension should only last a few minutes as your blood pressure adjusts to your new position. This type of low blood pressure tends to affect people more as they get older when it can lead to more frequent falls. Similar symptoms may also occur after exercise.

Postprandial hypotension

Your blood pressure can sometimes decrease (fall) after eating, causing dizziness, light-headedness, fainting and falls. This condition, known as postprandial hypotension, tends to occur more often in older people, particularly in those who have high blood pressure, or a condition such as Parkinson’s disease, or diabetes.

After a meal, your intestines need a large amount of blood for digestion. Your heart rate increases and the blood vessels in other parts of your body constrict (narrow) to help maintain blood pressure. If your heart rate does not increase enough, or if your blood vessels do not constrict enough to maintain blood pressure, your blood pressure will fall. This can then cause symptoms.

Throughout the day, your blood pressure can vary by between 30-40 mmHg (both systolic and diastolic) depending on what you are doing. Having a stressful week at work, the temperature outside, and even what you had for lunch could affect your blood pressure reading. 

Each time that you have your blood pressure measured, it is important that the test is carried out under similar conditions to ensure that the results are consistent. If you have a low blood pressure reading, your GP will first consider the everyday causes that might have affected it, before considering the possible underlying causes.  

Everyday causes

Many factors have a daily, or sometimes even hourly, effect on your heart and circulation. Below are things that could affect your blood pressure and, in some cases, may cause low blood pressure.

• The time of day - your blood pressure falls overnight so it will be low in the morning. 
• Your age - typically, blood pressure rises as you get older, although postural, or orthostatic, and postprandial hypotension are also more likely in the elderly. 
• How stressed or relaxed you are - if you are stressed, your heart will beat faster and your blood pressure will increase, and the opposite if you are relaxed. 
• How much exercise you do - initially, exercise will raise your blood pressure, but if you are healthy and exercise regularly, your blood pressure will be low when you are resting. 
• Your temperature - if you are cold, your heartbeat will slow down, and your blood pressure will fall. 
• If you have recently eaten - blood will be used for digesting food in your stomach, so the blood pressure elsewhere in your body will fall. 

Underlying causes 

If your blood pressure is still considered low after taking into account everyday factors such as those listed above, there may be another cause. Some possibilities are explained below. 

Medication 

Some medication may cause hypotension as a side effect. This tends to be orthostatic, or postural hypotension (low blood pressure when you stand up or change position). Examples of medication that can cause hypotension include:

• beta-blockers - these may be prescribed after a problem with your heart or hypertension (high blood pressure) 
• alpha-blockers - these are prescribed to lower blood pressure for people with hypertension 
• some antidepressants 

Your GP will discuss any possible side effects with you when prescribing medication. While you are taking medication, your blood pressure will be carefully monitored if you are considered to be at risk of hypotension.  

Serious illnesses or conditions

If you have an acute (short-term) illness, your blood pressure will be measured regularly because it is a good indicator of the severity of your illness. A heart condition, such as heart failure or a heart attack, can also cause low blood pressure, as your heart may not be able to pump blood around your body. 

Autonomic disorders

Autonomic disorders affect your autonomic nervous system and they can cause hypotension. Your autonomic nervous system is part of your nervous system (the network of cells that carry information around your body). It controls the bodily functions that you do not actively think about, such as sweating, digestion and the beating of your heart. 

The autonomic nervous system also controls the widening and narrowing of your blood vessels. If there is a problem with it, your blood vessels could remain too wide, causing low blood pressure. In particular, autonomic disorders tend to cause orthostatic hypotension.

Some examples of autonomic disorders are:

• diabetes mellitus - a long-term (chronic) condition caused by too much glucose (sugar) in the blood, 
• Parkinson’s disease - a chronic condition that affects the way the brain coordinates body movements 
• multiple system atrophy - a disorder that causes the brain signals to the muscles and limbs responsible for movement to deteriorate. 

Adrenal glands

The adrenal glands are two small glands that are located just above your kidneys. They produce hormones that control your blood pressure and maintain the balance of salt and water in your body. One of the hormones they produce is called aldosterone, which is responsible for controlling the amount of salt in your body.

If your adrenal glands become damaged - for example through an infection or a tumour - the production of aldosterone may be reduced, resulting in a loss of salt from your body. This can cause dehydration which, in turn, leads to low blood pressure. 

If a problem with your adrenal glands is diagnosed, it can be treated by increasing the amount of aldosterone in your body. This could also be a symptom of Addison’s disease (a condition in which the adrenal glands cannot produce enough of the hormones cortisol and aldosterone). Addison’s disease can also be treated with medication. 

Serious injuries and shock

Low blood pressure can also be caused by serious injuries or burns, particularly if you have lost a lot of blood. This can mean that there is less blood being pumped around your body. Low blood pressure can also occur if you go into shock after having a serious injury.

Other kinds of shock are described below.

Septic shock and toxic shock syndrome

Septic shock and toxic shock syndrome are caused by bacterial infections. The bacteria attack the walls of the small blood vessels, causing them to leak fluid from the blood into the surrounding tissues. This causes a significant drop in blood pressure (severe hypotension).

Anaphylactic shock

Anaphylactic shock, or anaphylaxis, is caused by an allergic reaction to something - for example, a wasp sting or a peanut. During an allergic reaction, your body produces a large amount of a chemical called histamine, which causes your blood vessels to widen and leads to a sudden, severe drop in blood pressure.

Cardiogenic shock

Cardiogenic shock occurs when your heart cannot supply enough blood to your body, so your blood pressure drops. This can happen during a heart attack.

Other causes

Other possible causes of low blood pressure are:

• Rare nerve conditions - if the nerves in your legs are affected, you may experience a severe drop in blood pressure when you stand up (postural or orthostatic hypotension). 
• Increasing age - as you get older, your arteries can become stiffer. If they do not constrict (get smaller), your blood pressure may drop, particularly when you stand up. 
• Pregnancy - during the early to mid stages of pregnancy, low blood pressure is fairly common. 
• Prolonged bed rest - low blood pressure may possibly occur as a result of moving less and having overall less nervous system activity. 
• Dehydration - low blood pressure may occur following particularly severe dehydration from vomiting and diarrhoea because the lack of water and salt in your body will reduce the volume of your blood. 
• Your genes - some research has suggested that low blood pressure is genetic. If your parents have low blood pressure, it is possible that you could inherit it from them.

Low blood pressure (hypotension) can be easily diagnosed by measuring your blood pressure. 

Measuring blood pressure

A blood pressure reading is taken using two measurements. The first measurement is known as systolic, which is the pressure in your arteries when your heart contracts and pushes the blood around your body. The second measurement is known as diastolic, which is the pressure in your arteries when your heart refills with blood in between heartbeats. Both systolic and diastolic blood pressures are measured in millimetres of mercury (mmHg).

Sphygmomanometer

Your GP, or practice nurse, will use a device known as a sphygmomanometer to measure your blood pressure. This device has an inflatable cuff and a scale of mercury, like a thermometer, as a pressure gauge. The cuff is placed around your upper arm and inflated to restrict the flow of blood in your arm. The air is then slowly released from the cuff. 

Your GP or practice nurse will watch the mercury pressure gauge and listen to your blood flow in the main artery of your arm using a stethoscope. Upon hearing your heart beat, the systolic pressure will be recorded. When the sound disappears, the diastolic pressure will be recorded.

Alternatively, a digital sphygmomanometer may be used. This measures your pulse using electrical sensors and takes blood pressure readings automatically. Blood pressure testing kits are also commercially available.

After you have had your blood pressure taken, your GP or nurse will give you your systolic reading first, followed by your diastolic reading. If your systolic blood pressure is 120 mmHg and your diastolic blood pressure is 80 mmHg, you will be told that your blood pressure is 120 over 80, which is commonly written as 120/80.

What is low blood pressure?

As a general guide, low blood pressure is a reading of 90/60 or less. However, it is not necessary for both your systolic and diastolic readings to be in this range for it to be considered low blood pressure. For example, a reading of 80/65 would be considered low because the systolic number is in the low range, and 100/55 would also be considered as low because the diastolic number is in the low range. 

If you have low blood pressure according to this guide, you do not need to worry. Having low blood pressure is considered healthy as it protects you from the risks and diseases of high blood pressure. You will only need to have treatment if you are experiencing symptoms as a result of your low blood pressure.

Postural or orthostatic hypotension

If your symptoms of low blood pressure mostly occur when you change position (postural or orthostatic hypotension), then your blood pressure may be measured before and after you move. For example, your blood pressure may be measured while you are sitting down and again while you are standing up.

Depending on what your seated blood pressure was, if your systolic reading drops by between 15-30 mmHg when you stand up, you may have orthostatic hypotension. 

Underlying causes

Your GP or practice nurse will usually be able to diagnose low blood pressure very easily. However, determining the reason for low blood pressure can be more difficult.

If you have an underlying condition that is causing low blood pressure, it is likely that you will have other symptoms as well. You should discuss these with your GP who may recommend that you have further tests.

If you have low blood pressure (hypotension), but you do not have any symptoms, you do not require treatment. If you are experiencing symptoms, your GP will try to establish the underlying cause of your hypotension in order to determine what treatment is necessary.  

Medication

If you are taking medication, and your GP suspects that it may be causing low blood pressure, they will probably recommend a change of medication or alter your dose. This includes medication to treat high blood pressure (hypertension) and medication to treat Parkinson’s disease.

Your blood pressure will be monitored while you are taking medication and any changes will be noted by your GP or practice nurse. If you are experiencing side effects from taking medication, you should discuss this with your GP. 

Underlying illnesses or conditions 

If your GP suspects that a disorder - such as a heart condition, adrenal gland failure or a nerve condition - is causing your low blood pressure, you may be referred to hospital for further tests and treatment.

If adrenal gland failure is found to be causing your low blood pressure, your GP may prescribe fludrocortisone to replace the missing hormone, aldosterone. This will usually be in tablet form and will need to be taken for life.

If a nerve condition is causing your low blood pressure, it can be more difficult to treat. You may be prescribed medication in order to help stimulate your nervous system.

Fluids and salt

Dehydration – when the water and salt content of your body is reduced - can cause low blood pressure. Increasing your fluid and salt intake can easily treat this. Ensuring that you drink enough fluid - at least eight glasses a day - will help with hypotension. This is because more fluids will increase the volume of your blood, and having more blood in your arteries will increase your blood pressure.

While people who have high blood pressure are usually advised to restrict their salt intake, if you have low blood pressure, you may be advised to include more salt in your diet. Your GP will be able to advise you about how much additional salt you need, and whether you can add salt to your usual food, or if you need to take salt tablets.

General advice

The following general advice will help to limit your symptoms of your hypotension, particularly postural or orthostatic hypotension. 

• Stand up gradually, particularly first thing in the morning. It may also be useful to try some other physical movements first to increase your heart rate and the flow of blood around your body. For example, stretching in bed before you get up, or crossing and uncrossing your legs if you are seated and about to stand.  
• Wear support stockings - sometimes called compression stockings. These are tight-fitting elastic socks or tights. They provide extra pressure to your feet, legs and abdomen, which will help stimulate your circulation and increase your blood pressure. 
• Raise the head of your bed or use extra pillows under your head. This will increase the flow of blood in your body and will also make it easier when you need to get up.  
• Avoid caffeine at night and limit your alcohol intake - this will help you to avoid dehydration, which can cause low blood pressure. 
• Eat small frequent meals, rather than large ones - this will help you to prevent postprandial hypotension (low blood pressure after you have eaten). Lying down after eating or sitting still for a while may also help. 

Very few people are prescribed medication for hypotension. The symptoms of hypotension can be usually be treated by making these small changes to your lifestyle and, in particular, by increasing your fluid and salt intake.

If medication is necessary, it will usually be medicines to expand the volume of your blood, or to constrict (narrow) your arteries. By increasing your blood, or decreasing your arteries, your blood pressure will increase, as there will be more blood flowing through a smaller space.